Separation of 6-aminocapronitrile and hexamethylenediamine from a mixture comprising hexamethylenediamine, 6-aminocapronitrile and tetrahydroazepine

ABSTRACT

The invention relates to the field of separation by distillation of 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD) from a mixture containing ACN, HMD, tetrahydrozaepine (THA), adiponitrile (ADN) and low boilers (LB). A method for producing a distillate stream comprising HMD is disclosed, which is suitable for the production of Nylon-6,6. The tails stream from the distillation of the mixture can be further distilled to produce a distillate containing ACN and THA, which is particularly suitable for use in the production of caprolactam and Nylon-6 from the caprolactam. Process conditions of the method of the invention disfavor the production of 2-cyanocyclopentylideneirnine (CPI).

FIELD OF THE INVENTION

The invention relates to the field of separation by distillation of6-aminocapronitrile (ACN) and hexamethylenediamine (HMD) from a mixturecomprising ACN, HMD, tetrahydroazepine (THA), adiponitrile (ADN) and lowboilers (LB). Also disclosed is a method for producing a distillatestream comprising HMD and a tails stream comprising ACN, THA and dimersof ACN and HMD that is further distilled to obtain a second distillatecomprising ACN and THA. The second distillate is essentially free ofdimers and is particularly suitable for use in the production ofcaprolactam since the low levels of dimers of ACN and HMD in the seconddistillate do not greatly affect the catalyst life in the caprolactamproduction process. Process conditions of the method of the inventiondisfavor the production of 2-cyanocyclopentylideneimine (CPI).

BACKGROUND OF THE INVENTION

It is well known in the Nylon industry that ADN can be hydrogenatedcatalytically to produce HMD by complete hydrogenation, or mixtures ofACN and HMD by partial hydrogenation. HMD can be used in the manufactureof Nylon 6,6. The hydrogenation reaction product also contains unreactedADN and unwanted byproducts such as THA. After hydrogenation, thereaction product must be refined, generally by methods involvingfractional distillation, and HMD and ACN must be separated from eachother.

It is also known that if the refining conditions involve too high atemperature, the unreacted ADN can isomerize into CPI. The CPI generallydistills with the ADN, and can form AMCPA(2-aminomethylcyclopentylamine) if it is recycled back to thehydrogenation reactor, which, if unseparated from the HMD, can causeinferior Nylon 6,6 to be made.

U.S. Pat. Nos. 6,346,641 and 6,462,220 teach distillation processes toseparate ACN and HMD in which the column temperatures are kept below 185degrees C.

However, neither of these patents teach methods that allow distillationto be performed in a manner in which HMD can be recovered substantiallyfree of THA.

U.S. Pat. No. 6,300,497 B1 teaches a method for reducing the THA contentof a THA/HMD mixture by distillation using column head pressures between0.3 and 3.0 bar, as well as reducing the THA content of a THA/ACNmixture by distillation using column head pressures between 0.1 and 1.3bar. U.S. Patent Application No. 2003 0023083 teaches a method forreducing the THA content of a THA/HMD mixture by distillation usingcolumn head pressures between 0.001 and 0.3 bar, as well as reducing theTHA content of a THA/ACN mixture by distillation using column headpressures between 0.001 and 0.2 bar. However, neither of thesedisclosures teaches a method in which a three component ACN/HMD/THAmixture is distilled so that the ACN and the HMD can be separated fromone another in such a way that a substantial portion of the THA remainswith the ACN, particularly when the three component ACN/HMD/THA mixtureis one that is derived from the product that is produced by the partialhydrogenation of ADN, such a product containing unreacted ADN that iscapable of being isomerized into undesirable CPI if distillationtemperatures in the refining train exceed about 195 degrees C.

U.S. patent application Ser. No. 10/383,947, now U.S. Pat. No.6,887,352, discloses a method for recovering HMD and ACN from a mixturecomprising HMD, ACN, THA, and ADN, the method comprising the steps (a)and (b) below:

(a) introducing a mixture comprising HMD, ACN, THA, and ADN into a firstdistillation column; separating as a group the HMD, ACN and at least aportion of the THA from the ADN, while minimizing isomerization of theADN into CPI; and then

(b) introducing the stream of HMD, ACN and the THA into a seconddistillation column and separating a distillate comprising HMD fromcolumn tails comprising ACN under conditions so that the THA separatesalong with the ACN in the tails.

Step (b) is accomplished preferably under column conditions of headpressure of at least about 200 mm Hg absolute and a pressure drop acrossthe column of greater than about 25 mm Hg. Under these columnconditions, however, there is significant formation of dimers of ACN andHMD as well as mixed dimers of ACN and HMD that separate along with theACN and the THA in the column tails. Presence of these dimers in the ACNand THA is believed to be detrimental to catalyst life in the process ofmaking caprolactam from the column tails.

It would, therefore, be desirable to have a method of producing amixture comprising ACN and THA that is substantially free of dimers ofACN and HMD as well as mixed dimers of ACN and HMD for use in makingcaprolactam without greatly affecting catalyst life. The presentinvention provides such a method.

SUMMARY OF THE INVENTION

The present invention provides a method of separating a feed mixturecomprising 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD),comprising:

-   -   introducing a feed mixture comprising ACN, tetrahydroazepine        (THA), and HMD into a refined distillation column;    -   operating the refined distillation column under conditions such        that:        -   a refined column distillate comprising HMD is withdrawn from            the refined distillation column; and        -   a refined column tails comprising ACN, THA and high levels            of dimers of ACN and HMD is withdrawn from the refined            distillation column;    -   introducing the refined column tails into a dimer removal        column; and    -   operating the dimer removal column under conditions such that:        -   a dimer removal column distillate comprising ACN and THA is            withdrawn from the dimer removal column; and        -   a dimer removal column tails comprising dimers of ACN and            HMD is withdrawn from the dimer removal column;    -   wherein the levels of dimers in the refined column tails        substantially exceeds the level of dimers in the dimer removal        column distillate.

The refined column distillate comprising HMD is suitable for use in theproduction of Nylon-6,6. The dimer removal column distillate comprisingACN and THA is suitable for use in the manufacture of caprolactamwithout greatly affecting catalyst life, which caprolactam can be usedin the production of Nylon-6.

A suitable feed stream for the refined distillate column can be preparedfrom the distillate produced by vacuum distillation of a mixturecomprising an ammonia-depleted mixture of HMD, ACN, THA, ADN, LB andhigh boilers (HB).

DETAILED DESCRIPTION OF DRAWING

The drawing consists of one Figure (FIG. 1) suitable for operating theprocesses of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown an apparatus 10 suitable foroperating the processes of the present invention. A feed mixture 12comprising HMD, ACN, THA and other low boilers (LB) is introduced into arefined distillation column 14. The head pressure should be in the rangeof about 200 to 760 mm Hg absolute. A preferred head pressure is 400 mmHg absolute. The column should be operated with a reflux ratio in therange of about 1 to 10. The column pressure drop should be in the rangeof about 25 to 300 mm Hg. Under these conditions HMD, LB and at most 10%of total THA in the feed are removed as a refined column distillate 16from the top of the refined distillation column 14. The refined columndistillate 16 preferably contains less than about 100 ppm THA. Operationof the refined distillation column 14 above about 200 mm Hg allows themajority of the THA to be preferentially removed in the refined columntails 18 along with ACN. If the head pressure is less than about 200 mmHg, progressively more THA partitions into the refined column distillate16.

Under the above operating conditions of the refined distillation column14, dimers of ACN and HMD are known to form. Chemical analysis showsthat a majority of the dimers thus formed remain with the refined columntails 18, which further comprises ACN and THA. Under the above operatingconditions, the refined column tails 18 contains less than about 1 wt %dimers. Less dimer formation may be observed at lower temperature andlower pressure operation, but the partitioning of THA is unfavorable atthese conditions.

The refined column tails 18 is further distilled in a dimer removalcolumn 22. The head pressure in the dimer removal column 22 should bemaintained below about 50 mm Hg absolute. Under these column conditions,a dimer removal column distillate 24 comprising substantially ACN andTHA separates from a dimer removal column tails 26 comprisingsubstantially dimers. The level of dimers in the dimer removal columndistillate 24 is negligible, making the dimer removal column distillate24 suitable for use in the production of caprolactam without greatlyaffecting catalyst life and thus suitable for use in the manufacture ofNylon-6 from caprolactam. Lower dimer content of the dimer removalcolumn distillate 24 results in improved catalyst life duringcaprolactam production.

The feed mixture 12 to the refined distillation column 14 can beproduced by distillation of an ammonia-depleted crude feed mixture 28comprising HMD, ACN, THA, ADN, LB and other high boilers (HB) in a crudedistillation column 30. The feed mixture typically contains about 1-90wt % ADN, 1-90 wt % ACN, 1-90 wt % HMD, <1 wt % THA, <2 wt % LB and <2wt % HB. In crude distillation column 30, HMD, ACN, THA and LB areremoved as crude column distillate 32 and ADN, HB and minor portions ofthe ACN, HMD and THA are removed as crude column tails 34. Crudedistillation column 30 is preferably a vacuum distillation column thatcontains structured packing. The crude distillation column 30 operatesat about 5 to 100 mm Hg absolute, preferably at about 60 mm Hg absolutehead pressure, with a reflux ratio of about 1.0. Operation of the crudedistillation column 30 at about 60 mm Hg absolute head pressure avoidsthe need for a column having an excessively large diameter. Thewithdrawal rate of crude column tails 34 should be adjusted to maintaina temperature below about 195 degrees C. Maintenance of the crude columntails temperature below about 195 degrees C. reduces isomerization ofthe ADN into CPI. CPI is undesirable since it can form AMCPA when therecovered ADN is hydrogenated, and AMCPA adversely affects the qualityof Nylon 6,6 made from HMD. Crude column distillate 32 can be used asfeed mixture 12 for the refined distillation column 14.

Crude column tails 34 can be further distilled in a tails distillationcolumn 36 in which ACN, THA and a minor portion of HMD are removed astails column distillate 38 and the major portion of ADN, CPI and HB, areremoved as tails column tails 40. Tails distillation column 36 ispreferably a vacuum distillation column containing structured packingand operating at a head pressure of about 10 to 60 mm Hg absolute,preferably about 20 mm Hg absolute, with a reflux ratio of about 1.0.All or a portion of the tails column distillate 38 can be reintroducedinto the crude distillation column 30. Operation of the crudedistillation column at a head pressure of about 20 mm Hg absolute allowsefficient separation of ACN from ADN without causing an undesirable hightemperature of the tails column tails 40, which can result in theformation of CPI

The ammonia-depleted crude feed mixture 28 can be made from a crude feedmixture comprising HMD, ACN, THA, ADN, LB, HB and ammonia by processingin an ammonia flasher (not shown). The ammonia flasher generallyoperates at atmospheric pressure and a person skilled in the art candetermine the operating conditions of a flasher given the feedcomposition.

EXAMPLES

The example below illustrates the invention as claimed herein, however,it is not intended to be limiting in any way.

Example 1

An ACN feed containing 0.62 wt % dimer was distilled in a 2-inchdiameter and 10-foot long distillation column packed with wire meshstructured packing. The feed was introduced in the middle of the column,5 feet from either end. The column was operated at 20 torr (2.67 kPa)head pressure and at a reflux ratio of 1.0. The column was equipped witha thermo-syphon reboiler with a capacity of about 1-liter. The columnwas operated in a semibatch mode. A distillate was removed from the topof the column. There was no tails draw in order to concentrate the dimerat the bottom of the column.

A total of 45 liters of ACN was fed to the column at a feed rate of 30ml/min. The amount of distillate collected was 43.5 liters. There was nodetectable dimer in the distillate as determined by gas chromatographicanalysis. The dimer content of the contents of the reboiler at the endof the run was 22 wt %. During the course of this run, the reboilertemperature increased from 140 degrees C. at the beginning of the run to155 degrees C. at the end, reflecting an increasing concentration ofdimers in the reboiler.

Based on these results, a dimer free ACN can be recovered from ACNcontaminated with dimer with an ACN recovery of greater than 97%.Greater recoveries can be achieved by further concentration of dimer inthe reboiler.

It is to be understood that the embodiments and variations shown anddescribed herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

1. A method of separating a feed mixture comprising 6-aminocapronitrile(ACN) and hexamethylenediamine (HMD), comprising: introducing a feedmixture comprising ACN, tetrahydroazepine (THA), and HMD into a refineddistillation column; operating the refined distillation column underconditions such that: a refined column distillate comprising HMD iswithdrawn from the refined distillation column; and a refined columntails comprising ACN, THA and dimers of ACN and HMD is withdrawn fromthe refined distillation column; introducing the refined column tailsinto a dimer removal column; and operating the dimer removal columnunder conditions such that: a dimer removal column distillate comprisingACN and THA is withdrawn from the dimer removal column; and a dimerremoval column tails comprising dimers of ACN and HMD is withdrawn fromthe dimer removal column; wherein the levels of dimers in the refinedcolumn tails substantially exceeds the level of dimers in the dimerremoval column distillate.
 2. The method of claim 1 wherein the headpressure of the refined distillation column is greater than about 200 toabout 760 mm Hg absolute and the pressure drop across the refineddistillation column height is at least about 25 to about 300 mm Hg. 3.The method of claim 1 wherein the head pressure of the refineddistillation column is greater than about 200 mm Hg absolute and thepressure drop across the refined distillation column height is at least25 mm Hg.
 4. The method of claim 1 wherein the head press tire of thedimer removal column is less than about 50 mm Hg absolute.
 5. The methodof claim 1 further comprising; introducing an ammonia-depleted crudefeed mixture comprising HMD, ACN, THA and adiponitrile (ADN) into acrude distillation column; and operating the crude distillation columnunder conditions to minimize isomerization of ADN into2-cyanocyclopentylideneimine (CPI) such that: a crude column distillatecomprising HMD, ACN and THA is withdrawn from the crude distillationcolumn; and a crude column tails comprising ADN, HMD, ACN and THA iswithdrawn from the crude distillation column; wherein the crude columndistillate comprises the feed mixture for the refined distillationcolumn.
 6. The method of claim 5 wherein the head pressure of the crudedistillation column is about 5 to about 100 mm Hg absolute.
 7. Themethod of claim 6 wherein the head pressure of the crude distillationcolumn is about 60 mm Hg absolute.
 8. The method of claim 5 furthercomprising: introducing the crude column tails into a tails distillationcolumn; operating the tails distillation column under conditions suchthat: a tails column distillate comprising HMD, ACN and THA is withdrawnfrom the tails distillation column; and a tails column tails comprisingADN and CPI is withdrawn from the tails distillation column; andreintroducing the tails column distillate into the crude distillationcolumn.
 9. The method of claim 8 wherein the head pressure of the tailsdistillation column is about 10 to about 60 mm Hg absolute.
 10. Themethod of claim 9 wherein the head pressure of the tails distillationcolumn is about 20 mm Hg absolute.